1. Field of Invention
The invention relates to an in-cylinder fuel injection type internal combustion engine which directly injects fuel into each cylinder, and more particularly to an ignition timing control of such an in-cylinder fuel injection type internal combustion engine.
2. Description of Related Art
In conventional port injection type internal combustion engines, fuel is first injected into a port in the form of a spray, and the fuel spray is then introduced into each combustion chamber during an intake stroke. Thus, the air-fuel mixture is evenly distributed in the combustion chamber and thereafter is ignited by a spark plug, namely so-called “homogeneous-charge combustion” is performed. In this case, the ignition of the air-fuel mixture does not largely depend on the position of the spark plug. During homogeneous-charge combustion, however, since the air-fuel mixture concentration is made almost constant in the entire area of the combustion chamber, the rarefaction of an air-fuel mixture is limited. To counter this, an in-cylinder fuel injection type internal combustion engine (hereinafter will be simply referred to as “a direct injection engine” where appropriate) which directly injects fuel into cylinders (combustion chambers) has recently been put into practical use. Having a capability of supplying (injecting) fuel into a combustion chamber at a desired time, in general, a direct injection engine performs so-called “stratified-charge combustion” where fuel is injected in a latter half of each compression stroke so that an air-fuel mixture having combustible concentration is produced around the spark plug, thus performing a so-called “lean burn engine operation.”
During the stratified-charge combustion, however, since the air-fuel mixture having a combustible concentration is produced only around the spark plug, the air-fuel ratio in the entire area of the combustion chamber tends to become much lower than the stoichiometric air-fuel ratio, which limits an ignitable region of the air-fuel mixture in the combustion chamber. In particular, it is difficult to create a region of an air-fuel mixture which is appropriate both when the load required of the engine is low and when the same load is high (Hereinafter, an engine operation performed when the load required of the engine is low will be referred to as “a low load engine operation”, and an engine operation performed when the same load is high will be referred to as “a high load engine operation” where appropriate). For example, since only a small amount of fuel is injected and a combustible region of an air-fuel mixture is therefore narrow during the low load engine operation, it is necessary to produce the air-fuel mixture around the spark plug. During the high load engine operation, conversely, a large amount of fuel is injected. Here, if the same ignition timing as that used during the low load engine operation is used, the spark plug ignites an air-fuel mixture having an excessively high concentration produced in the vicinity thereof, which results in a reduction in the combustion efficiency. Also, in the case where the direct engine is adapted to ignite the air-fuel mixture after it has been sufficiently dispersed within the combustion chamber during the high load engine operation, it may happen that the ignitable region of the air-fuel mixture has already moved away from the spark plug before the ignition time, and the ignition of the air-fuel mixture therefore may fail.
Also, when stratified-charge combustion is performed, a sufficient stability in combusting air-fuel mixtures may not be achieved owing to variations in the concentration of an air-fuel mixture among different regions of the combustion chamber. In consideration of these facts, presently, during the low load engine operation, stratified-charge combustion is performed by producing an air-fuel mixture and timing the ignition of the air-fuel mixture in a manner mainly suitable for the low load engine operation. During the high load engine operation, conversely, partial stratified-charge combustion or homogenous-charge combustion is performed.
Accordingly, it is significantly desirable that stratified-charge combustion be extensively performed during both low-load and high-load engine operations. Especially, the fuel economy improves if stratified-charge combustion can be performed also during high-load engine operations.